Quenching of Singlet O2 by O- and S-Centered Radicals
J. Am. Chem. Soc., Vol. 120, No. 2, 1998 401
thoroughly.3,51-54 Aside from chemical reactions, four mech-
anisms are generally cited: energy transfer, charge transfer,
paramagnetically induced intersystem crossing, and electronic
to vibrational energy transfer (EVET). Charge-transfer quench-
ing occurs between electrophilic singlet oxygen and species such
as amines and sulfides that have a nucleophilic lone pair. By
and large, rate constants are e109 M-1 s-1, and many cases
are 10 to 1000 times slower than that. This is not an especially
likely mechanism for peroxyl radicals, since they are electro-
philic species.2,3,55
Paramagnetically induced intersystem crossing due to electron
exchange56 has been demonstrated for a series of nitroxide
radicals, whose kq values did not show any significant solvent
effect, but did have a large steric dependence.15 Rate constants
of 105 to 107 M-1 s-1 were observed. It is clear that, like EVET
for diamagnetic molecules, this mechanism will exist as a
“baseline” for all organic radicals. What remains is to ask if
other, faster quenching mechanisms also exist.
Energy transfer is, of course, most directly characterized by
observation of the excited quencher. Perhaps no case is more
prototypical or well-studied than â-carotene, whose kq is just
Figure 4. Energies of the planar and T-shaped PhSO• conformers in
2A′′ and 2A′ states, relative to the planar 2A′′ state. Calculations are at
the MRMP2/6-31G(d,p)//ROHF/6-31G(d,p) level.
over 1010 M-1 s-1 3,57,58
Other sorts of energy transfer quenchers
.
include square planar metal complexes of Ni(II) and Co(II) and
various cyanine-, azomethine-, and chlorophyll-type dyes. This
assignment is generally made on evidence of a sufficiently low-
lying triplet state, and rate constants in excess of about 1 × 109
M-1 s-1. In the following paragraphs, the suggestion is made
that peroxyl radicals represent a new general class of energy
transfer substrates for singlet oxygen.
The first two entries of Table 1 demonstrate that the rate
constants kq for quenching of singlet oxygen by PhS• and PhSO•
are no greater than about 108 M-1 s-1. Because the other
initiators (aside from 2 and 3) produce one of these two radicals
along with the carbon-centered radical, and because PhS• and
PhSO• have previously been shown to react slowly with ground
state oxygen (i.e., reaction 5), only the peroxyl radicals generated
in entries 3-11 contribute significantly to the quenching of
singlet oxygen, as indicated in the table.
The solution phase transient absorption spectrum of PhS•
consists of a sharp band at about 300 nm and a broad band in
the visible, but is somewhat solvent-dependent. Emission
corresponding to both absorption bands has been observed at
low temperature47,48 and in the gas phase.49 Data for gas phase
and low-temperature nonpolar matrixes are in reasonable
agreement, and the 0,0 band of the lowest energy emission is
at 19 330 cm-1
.
In C2V symmetry, there are four states that could be calculated
by the method used in Table 2. However, there is no guarantee
that these comprise the four lowest states. The ground state of
2
PhS• is B2 (defining the phenyl ring as the xy plane), and a
CIS run was performed without symmetry to get a handle on
the makeup on the low-lying excited states. Similar results were
obtained as found in the literature with a different basis set:50
the first three excited states were composed almost entirely of
2B1, 2A2, and 2B2 configurations, respectively. The 2A2 and 2B2
states are very similar in energy at this level of calculation,
21 000 and 22 600 cm-1 above ground state. Tripathi cites
Raman and LIF data to argue that the broad visible absorption
is due to more than one electronic transition, promotion to the
In the case of the chlorides 2 and 3 (entries 12-15), it is
necessary to take into account that photolysis leads to the
production of atomic chlorine, as well as the carbon-centered
radical.28,29,35,59 Chlorine atoms abstract hydrogen from aceto-
nitrile with a rate constant of 1.3 × 107 M-1 s-1 59,60
.
The
2
2A2 and the second B2 state.50 The method used to calculate
subsequent addition of cyanomethyl radical to molecular oxygen
(reaction 5) produces the cyanomethylperoxyl radical with a
values in Table 2 is only applicable to the former, but it is
reasonably close to the 0,0 band of the lowest emission.
rate constant61 of 1.3 × 109 M-1 s-1
.
2
Thus the photolysis of Ph3CCl in acetonitrile leads to the
formation two peroxyl radicals: Ph3COO• and NCCH2OO•. The
kq value for this initiator is 2-3 times higher than that for 6,
In the B2 ground state, the unpaired spin of the radical is
almost entirely localized on the p-orbital of the sulfur atom that
is in conjugation with the phenyl ring. Computationally, there
is a low-lying 2B1 state, although there is no experimental
evidence for it as of yet. In this state it is the p-orbital whose
axis is rotated 90° with respect to phenyl conjugation that is
singly occupied. Unsurprisingly, the calculated separation
(51) Lissi, E. A.; Encinas, M. V.; Lemp, E.; Rubio, M. A. Chem. ReV.
(Washington, D.C.) 1993, 93, 699-723.
(52) Gorman, A. A. AdV. Photochem. 1992, 17, 217-274.
(53) Bellus, D. AdV. Photochem. 1979, 11, 105-205.
(54) Gorman, A. A.; Rodgers, M. A. J. In Handbook of Organic
Photochemistry; Scaiano, J. C., Ed.; CRC Press: Boca Raton, FL, 1989;
Vol. II, pp 229-247.
(55) Neta, P.; Huie, R. E.; Ross, A. B. J. Phys. Chem. Ref. Data 1990,
19, 413-513.
(56) Hoyntink, G. J. Acc. Chem. Res. 1969, 2, 114-120.
(57) Farmilio, A.; Wilkinson, F. Photochem. Photobiol. 1973, 18, 447-
450.
2
2
between the B1 and B2 states is small, 2600 cm-1
.
Discussion
The quenching of singlet oxygen by various molecules has
been studied for a number of years, and has been reviewed
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1984, 109, 31-34.
(60) Russell, G. A. J. Am. Chem. Soc. 1958, 80, 4997-5001.
(61) Neta, P.; Grodkowski, J.; Ross, A. B. J. Phys. Chem. Ref. Data
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Schuler, R. H. J. Phys. Chem. 1992, 96, 5344-5350.